The effects of thyrotropin-releasing hormone (TRH) on thyrotropin (TSH) subunit messenger RNA levels has been examined in various in vivo animal models. Despite major effects of TRH on TSH release, there were no effects on TSHAlpha or Beta messenger RNA levels. Similarly, there were no effects of TRH on thyrotroph number. Thus, factors other than TRH are responsible for the stimulation of TSH messenger RNA and thyrotroph growth.